Brake system having variable pump pressure control and method of pressure control

ABSTRACT

Electronic control of the supply pressure of the pump is suggested with respect to a hydraulic brake system which is used for brake slip control, traction slip control and further driving stability control operations. Means of action include, for example, the control or reduction of the motor current of the pump, electromagnetic closure of the suction valve of the pump or a brief change-over of the separating valve in the brake line.

BACKGROUND OF THE INVENTION

The present invention relates to a hydraulic brake system including apedal-operated master cylinder connected to a supply reservoir, a wheelbrake which is connected to the master cylinder by way of a brake line,a pump driven by an electric motor and connected to the brake line, andto a method of pressure control implemented in a brake system of thistype.

A brake system of this type is disclosed in German patent applicationNo. 41 28 386. The prior art brake system is used for brake slip controland traction slip control. During brake slip control operations, thebrake system operates according to the return principle. For tractionslip control operations, the pump of the brake system is of theself-priming type and adapted to take in pressure fluid from the supplyreservoir by way of a second suction line and the master cylinder. Aseparating valve in the brake line is an electromagnetically operatedvalve which is normally open and functions as a pressure limiting valvein its energized condition. When the supply pressure of the pump exceedsa predetermined value, as compared to the master cylinder pressure,pressure fluid discharges through the separating valve to the mastercylinder.

When a brake system of this type is also intended to be used for brakemanagement operations, to the effect of not only eliminating brake slipand traction slip but also for ensuring general driving stabilitycontrol, e.g. during cornering without braking, it is desirable that thesupply pressure of the pump is not fixed to a constant differentialvalue between the pressure line and the master cylinder, but is variablein conformity with requirements.

SUMMARY OF THE INVENTION

This objective is achieved by an electronically controlled adjustment ofthe supply.

Such adjustment operations can be performed by control of the motorcurrent of the electric motor, by an adjustable differential pressure ofthe separating valve in its operating position, or by limiting thepressure fluid supply to the suction side of the pump.

For example, the master cylinder pressure and the instantaneous, actualsupply pressure of the pump may be taken into account as measuredvariables for the evaluation.

Appropriately, the master cylinder pressure is sensed by pressuresensors which are arranged in the brake line between the master cylinderand the separating valve. The supply pressure of the pump can bedetermined, for example, by measuring the motor current and comparing itwith memorized motor characteristic curves. The resultant volume flowmay be converted to the present supply pressure of the pump by way of apressure volume pattern which is memorized in the electronic controlunit. The switching time of the outlet valve and the size of thelow-pressure accumulator are taken into account in these considerations.

The idea of the present invention will be explained in detailhereinbelow, making reference to the description of three Figures in thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 shows a brake system of the present invention.

FIG. 2 and FIG. 3 each show a segment of the brake system of the presentinvention, which can be inserted in FIG. 1 instead of the segment shownin dotted lines.

DETAILED DESCRIPTION OF THE DRAWINGS

In the brake system of FIG. 1, two identically designed brake circuits Iand II (of which only brake circuit II is shown) originate from a mastercylinder 1. The master cylinder 1 is connected to a supply reservoir 2and operated by brake pedal 3, with the result that braking pressure isbuilt up in brake circuits I and II. A brake line 4 originates from themaster cylinder 1 and extends through a separating valve 5. Brake branchlines 6 and 7 extend from the brake line 4 to one of wheel brakes 8 and9, respectively. Inlet valves 10, 11 are inserted in the brake branchlines 6 and 7, respectively. Return branch lines 12 and 13 extend fromthe wheel brakes 8 and 9 to a return line 16 to which a low-pressureaccumulator 17 is connected. Outlet valves 14 and 15 are arranged in thereturn branch lines 12 and 13. A first suction line 18 leads from thelow-pressure accumulator 17 to the suction side of a pump 19 whichincludes a suction valve 20 and a pressure valve 21. Pump 19 is aself-priming pump driven by a motor 22. A second suction line 23connects the suction side of the pump 19 to the brake line 4 between themaster cylinder 1 and the separating valve 5. A change-over valve 24 isinserted into the second suction line 23. By way of a pressure line 25,the pressure side of the pump 19 is connected to the brake line 4between the separating valve 5 and the inlet valves 10 and 11. A dampingchamber 26 including a subsequent throttle 27 is provided in thepressure line 25. At the level of the connection of pressure line 25 tothe brake line 4, further, a high-pressure accumulator 28 is connectedto the brake line 4 by way of an accumulator line 29. A switch-overvalve 30 is inserted into the accumulator line 29. The inlet valves 10and 11 and the separating valve 5 are configured as electromagneticallyoperated, normally open two-way/two-position directional control valves.The outlet valves 14, 15, the change-over valves 24 and the switch-overvalve 30 are electromagnetically operated, normally closed valves.

The operation of the illustrated brake system during brake slip controland traction slip control is disclosed in the state of the art. Fordriving stability control, no matter whether with or without theactuation brake pedal, the high-pressure accumulator 28 is connected toany one or both of the wheel brakes 8 and 9 by change-over of theswitch-over valve 30. This ensures a quick pressure increase, even atlow temperatures when the brake fluid is highly viscous. In theseoperations, the separating valve 5 is closed in order to prevent thepressure built up in the high-pressure accumulator 28 from propagatinginto the master cylinder 1 when the brake pedal 3 is not operated, andto thereby minimize reactions to the brake pedal 3 when the brake pedal3 is operated. In contrast to the state of the art, the separating valve5 (represented in dotted lines) has no pressure relief valve which opensfrom the pressure side of the pump 19 to the master cylinder 1. This isbecause control of the motor current of the electric motor 22 inconformity with requirements is provided for a variable adjustment ofthe supply pressure of the pump 19. To this end, the electronic controlunit 31 evaluates data about the master cylinder pressure sent bypressure sensors 32 and 33 in the brake lines directly arranged at themaster cylinder 1. The instantaneously prevailing supply pressure of thepump 19 is determined by measuring the instantaneous motor current,using a current measuring means 38, by comparing the value with motorcharacteristic curves memorized in the electronic control unit 31, andby converting the supply volume value produced by comparison to thesupply pressure by way of a pressure-volume pattern which is also storedin the electronic control unit. The rates of pressure fluid flow throughthe outlet valves at determined switching times and the size of thelow-pressure accumulator are taken into account in the conversion.Possible arrangements for the control of the supply pressure of the pump19 include, in addition to the control of the motor current, a temporarychange-over of the separating valve 5 and an electromagneticallyactuated closure of the suction valve 20. There is no specialillustration of the suction valve 20 for the sake of clarity.

Instead of the separating valve 5 in FIG. 1, it is also possible to usea separating valve 35 having a parallel-connected pressure relief valve36, as shown in FIG. 2 and known in the state of the art. The controlaction of the electronic control unit 31 occurs only if the mastercylinder 1 is operated by the brake pedal 3 and the pressure sensors 32and 33 sense a master cylinder pressure. The pressure is limited by thepressure relief valve 36 when the brake pedal 3 is not operated.However, it is also possible to adjust a defined maximum pressure by wayof the pressure relief valve 36, and the supply pressure of the pump 19can be limited to lower values by temporary change-over of theseparating valve 35.

The separating valve 37 of FIG. 2 combines the functions of theseparating valve 35 and the pressure relief valve 36 of FIG. 2. Thedeenergized basic position of the separating valve 37 permits avirtually unrestricted pressure fluid flow in both directions, and theenergized switch position of the valve is adjustable to a differentialpressure. The differential pressure between the pressure side of thepump 19 and the master cylinder, which has to be overcome, may be variedby way of a proportional magnet, for example. This obviates the need fora pulsed change-over of the separating valve. When the separating valve37 is used, control of the motor current or electromagnetic closure ofthe suction valve 20 is, at most, appropriate if pressure from themaster cylinder 1 is applied. However, it is also possible to adjust thedifferential pressure of the separating valve 37 to a correspondinglylower value when the brake pedal 3 is operated.

All of the above-mentioned control arrangements for limiting the supplypressure of the pump 19 are included in the present invention. They maybe used individually or jointly.

We claim:
 1. A hydraulic brake system including a pedal-operated mastercylinder connected to a supply reservoir, a wheel brake which isconnected to the master cylinder by way of a brake line and isconnectable to a low-pressure accumulator by way of a return line, aninlet valve in the brake line and an outlet valve in the return line, apump driven by an electric motor and connected to the low-pressureaccumulator by way of a first suction line and to the brake line betweenthe master cylinder and the inlet valve by way of a second suction line,the pump being connected to the brake line between the master cylinderand the connection of the second suction line by way of a pressure line,a separating valve in the brake line between the connections of thesecond suction line and the pressure line, a change-over valve in thesecond suction line and an electronic control unit,wherein the supplypressure of the pump during driving stability control is variablycontrollable by electronic actuation to adopt a desired variable valuedetermined from driving stability control parameters being irrespectiveof brake pedal actuation during periods without brake slip control ortraction slip control for increased driving stability control.
 2. Thebrake system as claimed in claim 1, wherein the motor current of thepump is controllable based on comparing motor current data to mastercylinder pressure data.
 3. The brake system as claimed in claim 1,wherein the separating valve is variably adjustable to a differentialpressure which is variable.
 4. The brake system as claimed in claim 1,wherein the suction valve of the pump is adapted to be closedelectrically.
 5. The brake system as claimed in claim 1, wherein apressure sensor is interposed in the brake line between the mastercylinder and the separating valve.
 6. The brake system as claimed inclaim 1, wherein a means for measuring the motor current is provided.